Circuit boards are used in the membrane switches used in personal computer keyboards, seating sensors, pressure sensors and the like. Some of the circuit boards are composed of a plastic film and an electroconductive layer obtained by printing an electroconductive paste for electron beam curing on the plastic film and curing the paste by irradiating with an electron beam.
An electroconductive paste that uses a radical polymerizable resin such as an acrylate compound is known as an electroconductive paste for electron beam curing used to form these circuit boards (Patent Documents 1 and 2). Radical polymerizable resins cause an increase in the crosslink density of the electroconductive paste and increase curing shrinkage ratio. In addition, when an electroconductive layer is formed by coating an electroconductive paste for electron beam curing containing a radical polymerizable resin onto a plastic film and curing by irradiating with an electron beam, sufficient hardness can be imparted to the electroconductive layer.
On the other hand, an electroconductive paste obtained by mixing a cationically polymerizable resin into a radical polymerizable resin is known as an electroconductive paste for electron beam curing (Patent Document 3). Since this electroconductive paste for electron beam curing allows the proportion of radical polymerizable resin to be reduced, curing shrinkage during the curing process can be inhibited.